The use of capacitors is known for the effective storing of electrical charges. The capacitance of a capacitor with two parallel electrodes is known to be approximately proportional to the dielectric constant of the medium between the two electrodes, proportional to the overlapped area of the two electrodes, and inversely proportional to the gap distance between the two electrodes. The capacitance changes when a displacement of one electrode with respect to another changes the overlapped area and/or changes the gap distance. Those skilled in the art applied these scientific principles (or relationships) and fabricated various capacitive sensors including displacement (position) measuring ones.
Based on the above scientific principles, a displacement can be measured in two different ways. One method is detecting capacitance variation due to a gap variation between two parallel electrodes, where the motion is perpendicular to the electrode surfaces. Because a capacitance of a capacitor is extremely sensitive to its gap variation, particularly when the gap is small, this kind of method is found in nano-positioning applications. Those sensors can sense a displacement as small as a few picometers, while they are limited to a measurement range of a few hundred micrometers. Another issue for this kind of sensor is nonlinearity due to the inversely proportional relationship between the capacitance and the gap distance of a capacitor, although this problem can be resolved with a digital linearization technique.
Another method is detecting a capacitance variation due to a size variation of an overlapped area between two parallel electrodes, where the motion is parallel to the surfaces of the electrodes. A commercially available sensor based on the area varying principle is desirable, but hard to find. Implementing the area varying principle perfectly in practice has proven to be a significant challenge. It is very difficult to measure the size variation of an overlapped area by having a motion in a parallel direction to the electrode surface without any variation of the gap distance due to motion perpendicular to the electrode surfaces. As mentioned above, the capacitance of a capacitor is extremely sensitive to its gap distance. It can be estimated, for example, that increasing or decreasing a capacitor gap of 100 micrometers by one micrometer, creates a capacitance variation of one percent. A 100 micrometer gap is in the practical range while a one micrometer tolerance for mechanical moving parts is quite a challenge for economical mass production. The one percent uncertainty error is unacceptable in terms of accuracy for most applications. As a result of difficult gap variation control, a sensor based on area variation is impractical unless useful compensation techniques are developed to reduce influence of gap variation.
There is another challenging problem for an area varying capacitive sensor to have a both highly sensitive and long range measurement. An area varying capacitive sensor cannot have a long range measurement because of the cyclic nature of its output signal. Transition points exist where a signal changes from increase to decrease or vice versa. The measurement uncertainties near the transition points are usually too big to be acceptable in terms of accuracy for most applications. This makes a sensor based on area variation impractical for long range measurement unless a solution is found to eliminate the uncertainties.